1. Field of the Invention
The present invention relates to filters in gas flow systems in general and particularly to adsorption and/or absorption filters in anesthetic or sedation delivery systems.
2. Description of the Prior Art
Inhalation anesthetics are drugs that are breathed into the lungs and from there absorbed into the blood. The anesthetic blocks the perception of pain and other sensations, allowing patients to undergo surgery, sedation or other procedures without the distress and pain they would otherwise experience. To support the administration of anesthesia, anesthetic or sedation apparatus are used. In anesthetic apparatus, fresh breathing gas is mixed with highly concentrated anesthetic from an anesthetics evaporator in a conduit, leading into and out of the patient.
Some anesthetic delivery systems are particularly adapted to re-use anesthetic that is not assimilated by a patient from previously inhaled anesthetic dose. An example of such a system is disclosed in U.S. Pat. No. 5,471,979, a so called anesthesia reflector system. During exhalation, the exhalation gas passes through an adsorption and/or absorption filter wherein the anesthetic that is not assimilated by the patient is adsorbed and/or absorbed, while the majority portion of the exhalation gas passes through the filter and is evacuated from the system. During inhalation, the adsorbed/absorbed anesthetic is desorbed from the adsorption and/or absorption material in the filter, and is re-supplied to the patient. That is, the adsorption and/or absorption filter operates as a reflector of anesthetics.
When it is time to wake up the patient, or if some patient related parameter indicates that the level of anesthetic/sedation is too deep, it is desirable to cut off the supply of anesthetic quickly. This is, however, impossible in reflector anesthetic delivery systems since some medium remains in the filter. When the supply of anesthetic to the patient must be interrupted, there are mainly two alternatives depending on how urgent the patient's need for fresh breathing gas is. The first alternative is to gradually air out the anesthetic but this takes time and in the meanwhile the patient receives an undesired supply of the medium. The second alternative is to remove the entire filter unit from the respiration circuit, as is known in the Hudson Anaconda™ sedation system. The filter unit removal is accomplished manually and since the tubes attached to the filter unit must be disconnected, the gas flow circuit is broken, resulting in that some of the anesthesia agent inevitably will be discharged into the surrounding air, potentially exposing the medical personnel. It has also the drawback that an undesirable decrease in pressure in the patient's lungs can occur.
To be able to cut off the supply of anesthetic quickly, without breaking the gas flow circuit, WO 03/090826 presents an anesthetic delivery system with an advanced air-regulation system comprising several different flow sections of which one directs the inhalation gas in a manner that makes it possible to bypass the adsorption and/or absorption filter. This solution, however, is technically advanced which increases the risk of technical hitches, takes up a great deal of space and is relatively expensive.
U.S. Pat. No. 6,488,028 discloses an anesthetic device where the problem regarding interruption of the anesthetic supply is solved by a rotatable filter which can be rotated between two positions with the aid of a turning element on the outside of the filter housing. When arranged in the first position, the gases breathed are forced to pass through the filter in a first gas passage while the filter, when rotated to the second position, obstructs said first gas passage, forcing the gases to bypass the filter in a second gas passage between the filter and the inside of the housing. However, this solution exposes the filter to the passing gas flow and there is a high risk that a portion of anesthetic is undesirably desorbed at the filter surface, particularly if the filter is fully saturated with anesthetic agent. This makes this solution ineffective and inappropriate for many applications.
There is a desire to be able to arrange the adsorption and/or absorption filter in a way that makes the re-use of anaesthetic effective and controllable. It is further desirable to be able to cut off the supply of anaesthetic, i.e. disconnect the anaesthetic reflecting filter, in a quick and simple way, without breaking the gas flow circuit. Further, one would like to avoid cabling and technically advanced tubing and valve solutions in the patient's immediate proximity. It is also a desire to achieve a safe, inexpensive and easy to use solution compared to the prior art.